Please Tell Me What You Think of This Excerpt of My Story?

Silent as a cheetah hunting for its prey, I slipped into the cool September night, illuminated only by the handful of stars that survived the multitude of artificial lights in abundance around the city. Gosh, cities who would ever want to live in an overpopulated land full of inorganic items, just taking up space that originally belonged to nature. At least I’d been stationed in a metropolitan area in the suburban part, with a bit more trees and animals than the big cities. Just as I let my thoughts wander off I heard it, the snap of a twig. I took a deep breath in and scolded myself for not being more careful, after all, this was forbidden according to Ema, and what Ema said went. Even so, I just had to investigate, no matter which rules I’d have to break in the process.
I started being extra-careful now, sometimes even elevating my feet when I saw leaves in my path, I was now officially out of the “common ground” area which we were allowed to hang out in at times. Now I had entered the families’ private property, filled with their fancy equipment to keep everyone out. Too bad they didn’t bother adding that equipment to the backyard, as there was only a fence to keep intruders out from the back. They probably wouldn’t suspect that anyone could be living in the woods behind their home, or that the “woods” went back about fifteen feet and then it was an area of fewer trees and more hills, and also a home where many people lived.
I let out a giggle as I climbed over the fence and jumped down into their backyard. Wow, I had never actually been inside this area, it was a vast area of land filled with a garden, tennis courts, a pond, a swimming pool, and a house about the size of mine back in the forest. Luckily, thanks to Ema’s training, we had all learned that at this particular families estate, they owned their own big house (which she described as a mansion) and a guesthouse, which was usually vacant. I sneaked around the large expanse of land, wondering how a family of five could need so many things. I hadn’t even noticed the small basketball court or the miniature second pool. I knew the father of the family was some sort of doctor, but I hadn’t realized how much money they must make until I got a good look around their property. Ema did tell me that this family was “rich”, so now I got a good idea on what “rich” really was. I was impressed, but I did have more important things to worry about. I began to stealthily creep towards the mansion, always cautiously checking the area.
Now I had to be even more nervous than earlier, the mansion was so close now! If anyone was looking out of their window they would definitely catch a glimpse of my silhouette and assume I was a criminal. At that point I wouldn’t even be able to escape because they’d find out what I was, and that was beyond imagination. I involuntarily shuddered just at the image of them finding out about us. That would be a disaster in the making, to say the least.
I was now tiptoeing towards the hidden walkway between the mansion and the garage, Ema had told me that was where cars were kept. That was another item that disgusted me, cars. Those things were one of reasons why we had to be in this realm to begin with. There’s actually a huge list of reasons were here, but the general understating was that we were to “observe the human race in all aspects of life, report findings, and (in some cases) interfere and offer better ideas to save their race.” Without us, humans would’ve died off in about 1,000 years, with all their violent tendencies and lack of common sense.
Craaaack, I stepped on another snap, I really need to stop day (or night) dreaming and focus on my present predicament. Okay, so the walkway is right there, and once I pass through that I’ll be in front of the mansion and my destination will be so close. I had to prepare for a second though, after all once I got in front of the mansion cars driving on the street would be able to see me clearly. Not to mention the whole family at the mansion, which is 5 whole people and…suddenly I break out into a cold sweat. Focus, I tell myself, all you have to do is go to the mailbox, grab some “junk” mail (who knows why’d they call it junk, it’s quite fascinating), and then sprint back to the hideout. Slowly but surely, I find myself walking through the walkway, sneaking towards the mailbox, and I quickly reach in and grab the first papers that touch my hand. I’m just about to run back the way I came when I see a large blob of light coming up from behind me, than I hear the sounds of a car quickly approaching. I failed, I think to myself. I see a man immediately exit a large car, calling out “Who’s there? Who?!” In a split-second I decided that the only way I’ll be able to make it out of this situation is to sprint.

Rating: 7/10

The word choice was nice, but make sure you don't sound like you're throwing up a thesaurus. Such a glaring flaw in any book. It makes you look like an amateur trying to SOUND professional. And LOTS of the sentences were way too wordy.

Also: "Focus, I tell myself…." and "I decided that the only way to make it out of this situation is to sprint."

See something wrong here? Stick with one tense, not flip back and forth.

Overall, it seems like you're a young writer trying to sound like you're not, and even though you seem to have talent, those flaws makes you seem even more inexperienced.